You ran a speed test, or you pasted your homepage into a tool like imagecheck.tools, and got a nudge to serve images in a "next-gen format." Nine times out of ten that means switching JPEG files...
You uploaded a photo, hit save, and your CMS threw back a warning: "This image is too large." Or you ran your homepage through PageSpeed Insights and got flagged for slow-loading images. Either...
The question to ask before writing any alt text, what to write for photos, products, charts, logos, icons and linked images, when to leave it empty, and the rules the checker applies.